1. Field of the Invention
The present invention relates to an optical device having a reflection preventing film, such as a lens or a filter.
2. Description of the Related Art
A known reflection preventing film of an optical device, such as a lens, which requires reflection prevention is a multi-layer film formed by alternately laminating a highly refractive substance, such as TiO.sub.2 or ZrO.sub.2, and a low-refractive substance, such as MgF.sub.2, by a vacuum vapor deposition method or a liquid applying method. The layer arranged close to the air must be formed of a substance having as low a refractive index as possible. According to the conventional art, this layer is formed of MgF.sub.2, which has an nd=1.38 (nd being a refractive index with respect to 587 nm ray). Although crystalline fluorocarbon resins have lower refractive indexes than MgF.sub.2 ; for example, polytetrafluoroethylene has an nd=1.35, they have poor formability and very low solubilities in general organic solvents. Therefore, according to the conventional art, crystalline fluorocarbon resins has not been used to form thin films such as reflection preventing films.